I still use ITA Matrix which is not perfect when it comes to A3, but helps to narrow down possibilities.

I haven't found anything good this summer. I suspect that the planes are more filled up so they don't need to offer special deals. I haven't looked at winter yet.

If a return is expensive, try one-ways and then get a low-cost flight back to TXL, and do this twice.

This year I did CPH-ATH-LHR and LHR-ATH-LHR. Next year I am just going to do LHR-ATH-SKG return.

Actually I find the A3 flights to be the easy bit. It's harder to get 12000 miles (or 7000 since the A3 flights give you something) without a long-haul. My long-hauls are on oneworld so I had to do 12 SK flights this year, and am kind of fed up with CPH now